This invention relates to improvements in interactive compositions and to methods for producing microwave interactive structures. In one of its more specific aspects, it also relates to microwave interactive coatings.
The cooking of food and heating of other microwave interactive substances by microwave radiation has become increasingly popular and important in recent years because of its speed, economy, and low power consumption. One of the major drawbacks of microwave ovens for preparation of foods has been their inability to brown or sear the food product to make it similar in taste and appearance to conventionally cooked food. Crisping of certain foods, e.g., the crusts of pies or pizzas, or the surface of fish fillets, potato cakes, or the like, may be accomplished only by means of special packages or utensils.
Several methods have been attempted in the prior art to overcome the browning and crisping problems. One such method for browning or crisping foods or for concentrating heat to produce superelevated temperatures in selected areas of packages involves the use of a metallized coating on paperboard. In some instances, metal particles are vacuum deposited onto a film, preferably a polyester film and the metallized film then laminated onto paper or paperboard. The thus metallized paper or paperboard can then be used in a microwave oven to heat, brown or sear food items placed on the metallized paperboard or in packages comprising metallized paperboard. While such structures can be very effective as heaters, the vacuum deposition process does not readily lend itself to coating selected areas of the film or the application of varying amounts of metal particles or metallized coating to selected areas of the finished carton or structure.
Other methods of forming microwave responsive heating elements involve dispersing particles which are lossy microwave energy absorbers, e.g. carbon, metals, and metal alloys, in a binder and forming a sheet or laminate comprising the dispersed particles. Turpin, U.S. Pat. No. 4,190,757 discloses heaters which comprise a layer of particulate lossy materials, e.g. carbon, iron and various metal oxides, or alloys, in an inorganic binder, e.g. sodium silicate. U.S. Pat. No. 4,264,668 to Balla discloses a heating element made up of a layer of carbon black in an acrylate binder laminated between a carrier layer, e.g. paper, cardboard or plastic material, and an outer layer of thermoplastic material. European Patent Application Publication No. 242,952 discloses a heating element made up of a metal or metal alloy in flake form in a thermoplastic dielectric matrix on a dielectric substrate.